1. Field of the Invention
This invention relates to optical fibers and optical fiber amplifiers.
2. Discussion of the Related Art
Some optical amplifiers use a doped optical fiber to perform optical amplification. The doped amplifier fiber receives both multi-mode pump light and a single-mode input optical signal for amplification. One arrangement introduces both the pump light and the input optical signal into one end of the amplifier fiber. Since the pump light and input optical signals come from separate fibers, these fibers are often combined into a bundle prior to coupling to the amplifier fiber. The fiber bundle improves the coupling with the amplifier fiber.
FIG. 1 shows a portion of a fiber amplifier 10 that uses a conventional fiber bundle 12. The fiber bundle 12 optically couples through end 13 to amplifier fiber 14, e.g., a multi-modal fiber having erbium and/or ytterbium dopants. The fiber bundle 12 incorporates two or more pump fibers 16-21 and input fiber 24. Each pump fiber 16-21 receives pump light at one end from a single laser diode 27-32 and transports the light by total internal reflection to end 13, which couples to the amplifier fiber 14. The input fiber 24 delivers the optical signal to be amplified. Near the end 13, fibers 16-21, 24 of fiber bundle 12 have been bonded together.
FIG. 2 is a cross-sectional view of the end 13 of the fiber bundle 12 shown in FIG. 1. The input fiber 24 is a single-mode optical fiber located at the center of the fiber bundle 12. The pump fibers 16-21 are multi-mode optical fibers located around the circumference of the input fiber 24.
The complete fiber bundle 12 and amplifier fiber 14 may have different outer diameters. If the diameters differ, a region 26 of the fiber bundle 12 adjacent the joint end 13 is tapered so that the fiber bundle""s diameter matches that of the amplifier fiber 14 at the joint. Matching the outer diameters improves light transfer from the fiber bundle 12 to the amplifier fiber 14.
The amplifier fiber may have an outer core with a polygonal cross section that enhances the coupling of pump light to the optically active dopants.
Various features of the fiber bundle 12 and amplifier fiber 14 improve optical couplings in fiber amplifier 10. But, still better optical couplings are desirable to push amplifiers to even higher optical gains.
In one aspect, the invention features a manufacture. The manufacture includes a first optical fiber and a plurality of second optical fibers. The second optical fibers have cross sections with aspect ratios of two or more. Distal sections of the fibers form a bonded structure. In the bonded structure, each distal section bonds along a length to another one of the distal sections and to the first optical fiber.
In a second aspect, the invention features an apparatus. The apparatus includes a plurality of laser diodes and a fiber bundle. The fiber bundle includes a first optical fiber and a plurality of second optical fibers. The second optical fibers have cross sections with an aspect ratio of two or more. Distal sections of the fibers form a bonded structure in which each section bonds along a length to another one of the sections and to the first optical fiber. Each laser diode is optically coupled to one of the laser diodes.
In a third aspect, the invention features a process for amplifying an input optical signal. The process includes transmitting pump light from laser diodes into associated pump optical fibers. The fibers have cross sections with aspect ratios of at least two. The process also includes delivering the transmitted pump light to one end of an amplifier optical fiber and transmitting the input optical signal into one end of the amplifier optical fiber.